Process for producing mica pulp



May 24, 1955 A. J. G. BOUCHET 2,709,158

PROCESS FOR PRODUCING MICA PULP Filed June 12, 1951 I-IEAT TREATMENTPARTIAL DEHYDRATION OF ABOUT 800C. WATER OF CRYSTALLIZATION COOLING INGAS ENTERS BETWEEN GASEOUS MEDIUM MICA LEAVES IMMERsIoN IN LIQUID TRAPSGAS LIQUID BETWEEN LEAVES REDUCTION OF PRES URE GAS EXPANDS AND AND SOPENS UP LEAVES AGITATION WHILE VAGITATION FORMS IN LlQUlD PULP I MICAPULP I Fig. I

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United States Patent PROCESS FOR PRODUCING IHICA PULP Antoine JeanGeorges Bouchet, Paris, France, assignor to Samica Corporation, Dover,DeL, a corporation of Delaware Application June 12, 1951, Serial No.231,271

4 Claims. (Cl. 252- 378) This invention relates to the production ofmica pulp and more particularly to an improved process for pro ducing amica pulp capable of forming mica paper and molded articles wherein micaflakes cohere without necessity for bonding agents. The mica pulpproduced by the process of this invention is elsewhere described inLetters Patent No. 2,549,880 to Jacques Jules Bardet.

The process described in the above-mentioned Bardet Patent No. 2,549,880consists essentially in first heating a batch of blocks and pieces ofnatural mica to a temperature of about 800 C. for sufiicient time toeliminate therefrom a portion of the hydroxyl ions in the molecule. Thisheat treatment had the eifect of slightly changing the visual appearanceof the mica and also of expanding the pieces of mica. While the mica wasstill hot, it was quenched in a concentrated soda solution, followingwhich the soda was drained off and the mica then immersed in a solutionof sulphuric acid. This sequence of steps brought about a partialsoftening of the mica and a cleavage of the leaves or scales of the micapieces. The soda entered the interstices between the mica leaves duringquenching, and thereafter the sulphuric acid reacted with the soda toproduce gas between the said leaves and gradually to force them apart.After the acid bath, the mica was in the form of substantially finelydivided, thin flakes with a few blocks remaining in an expanded andexfoliated condition. The next steps in the original process includedwashing out the salts formed by the acid soda reaction, and thereafterintroducing the mica into an agitator which completed the finaldisintegration of remaining pieces. After agitation, the mica was in anextremely finely divided state of suspension resembling a pulp and couldeither be dried into cake form for shipment or else employed immediatelyin a paper-making or molding process.

While this process was satisfactory, there were certain disadvantagesinherent in its practice. In the first place the use of sulphuric acidwas somewhat objectionable as requiring special acid-resistingequipment. Secondly, the process resulted in the formation of largequantities of salts which had to be totally eliminated and consequentlytended to pollute the adjoining rivers into which the efiiuent wasdischarged. Thirdly, and perhaps the most significant, disadvantage inthe original process was the fact that it required numerous stepsincluding repetitive removal of batches of mica from baths,transportation of the same, changes of liquid media, etc. This excessivehandling of the mica was regarded as undesirable because of certainminor losses of mica from step to step, and also because of the addedlabor costs incident to each step of the process.

in searching for a more efiicient and simpler process, the previouslyknown methods for treating mica were of little avail. It must be bornein mind that after heat treatment to about 800 C. and partialdehydration, mica becomes noticeably limp, and for that reason theconventional processes of grinding, rubbing, pounding, or other violenttreatments of mica for separation thereof 'ice which converted the micainto a dust could not be employed to produce the pulp described in theBardet Patent No. 2,549,886. Such violent working of the mica after thesaid heat treatment did not provide a product lit for formation intocommercial sheets or articles.

Therefore it is an object of this invention to provide a process forproducing mica pulp, which may be carried out with a minimum number ofsteps and without constant shifting of batches of mica from one liquidmedium to another. It is a further object of my invention to provide aprocess for producing mica pulp, which is more suited for continuousoperation and in which efliciency may be improved by permitting recoveryof heat by the elimination of quenching steps. An additional object ofmy invention is to provide a process which accomplishes the foregoingobjects without subjecting the mica to violent or destructive action andthereby permits the production of a good quality of mica pulp.

in the accomplishment of these and other objects of my invention, in apreferred embodiment thereof, I commence by heat treating natural micato a temperature of about 860 C. and for sufiicient time to eliminateapproximately one-half of the hydroxylions therefrom and to expand theindividual blocks of mica. Following this heat treatment 1 permit themica to cool, at which time the retained heat in the mica may be partlyrecovered by conventional heat exchange mechanism. Following cooling, Iintroduce the heat treated mica into a liquid medium which may be thesame liquid medium employed in the final paper-mal ing or molding stagesfor the pulp. The said medium is preferably water, but may be otherliquids or emulsions provided they do not inhibit agglomeration of themica flakes during the formation of sheets or other articles. While themica is in this medium 1 reduce the pressure over the surface of thesaid medium to expand the gas retained between the leaves of the micaand thereby promote exfoliation of the same. Simultaneously I alsomildly agitate the mica in the same liquid medium thereby bringing abouta mechanical exfoliation and disintegration of agglomerates along withthe exfoliation which is caused by the expansion of the gas between themica leaves.

To assist in an understanding of my invention, 1 have provided theaccompanying drawings in which:

Fig. 1 is a flow diagram of the process of my invention;

Fig. 2 is a view in side elevation of the vat adapted for low pressureand agitation of mica in the liquid medium; and

Fig. 3 is a sectional view along the lines 3-3 of Fig. 2.

It will be understood that permitting the heat treated mica to cool inthe air or other gas for a substantial time permits air to penetratebetween the mica leaves which have been partly opened up by the heattreatment. Thereafter when the mica is immersed in the liquid medium, alarge part of the air between the leaves remains there and its expansionis accomplished by reducing the pressure of the said medium to a valuesubstantialiy below the pressure of the air between the leaves. This maybe done either by subjecting the mica to high pressure in a gaseousmedium prior to immersing the mica in the liquid, and thereafterreleasing the pressure; or else it may be done by maintaining the micaunder atmospheric pressure after cooling and by then drawing a vacuumover the liquid medium after the mica has been immersed therein.

A word of caution is deemed necessary with respect to the temperature towhich the mica is heated in order to obtain partial dehydration.Although it may be that different types of mica require slightlydifferent temperatures, the figure of 800 C. is regarded as the bestapproximation. However, the volume of the particular batch of micaundergoing treatment will to some ex tent determine the amount of heatsupplied to it from the exterior. It must be remembered that mica is anexcellent thermal insulator; consequently it will require a longer timefor heat to penetrate the interior of the mass of the batch, the timeincreasing with the size of the mass. At the same time if excessive heatis applied to the interior of the mass there is danger of completedehydration of the surface portions thereof which results in completepulverization of the said surface portions and consequently a poor pulp.With the figure of 800 C. as the average optimum, those skilled in theart will readily recognize that the desired dehydration may be obtainedeither by excessive heating for a short period of time, the time beinginsufficient completely to dehydrate a significant portion of the mica,or the heat treatment may be carried out at a lower temperature for alonger period of time to obtain thorough penetration of the heat intothe interior of the mica without completely dehydrating any of it. Thisis not to say that the desired dehydration could be obtained by heatingthe mica for an indefinite period at, say, 400 C. because it hasdefinitely been established that a temperature in the generalneighborhood of 800 C. is required if the desired result is to beobtained. However, heating for several hours at 750 C. will often provethe practical equivalent of heating for a few minutes at 850 C. For thesake of convenience we have chosen the expression approximately 800 C.to describe the heating step within the general limits of time andtemperature as discussed herein.

Water is definitely the preferred liquid medium for carrying out myinvention. However, I have found that other aqueous solutions andemulsions of, for example, melamine formaldehyde, have beensatisfactory. Exfoliation may be accomplished in still other liquidmediums such as gasoline, benzene, anhydrous alcohols, and the like,where the end use of the pulp may be for purposes other than makingpaper. However, if such latter-mentioned mediums are employed, andthereafter it is desired to prepare the pulp for making mica paper, thesaid medium need only be washed out and replaced by an aqueous medium.

The mechanical agitation of the mica pulp is carried out in a largecircular vat adapted for the previously described pressure dilferentialas indicated at 12 where a vacuum is drawn and provided with a propeller14 driven by a motor 16. The propeller 14 circulates the mica from apoint near the bottom and center of the vat 10. The fully exfoliatedmica enters into suspension and becomes distributed throughout the waterin the vat, while the still partially agglomerated cakes remain nearerthe propeller at the bottom. The pulp in suspension is drawn off fromthe top of the vat at 18. In this manner the lighter and more fragilemica flakes do not receive destructive agitation treatment.

From the foregoing description it will be seen that the process of myinvention may be carried out with a minimum of steps. Agitation andexfoliation are accomplished simultaneously without excessive treatment,and the constant changing from one medium to another 4 has beeneliminated because only one liquid medium is necessary.

Various minor variations of the process of my inven tion will now beapparent to those skilled in the art and therefore it is not intended toconfine the invention to the precise form of the preferred embodimentherein shown but rather to limit it in terms of the appended claims.

Having thus disclosed and described an illustrative process of myinvention, what I claim as new and desire to secure by Letters Patentis:

1. A process for producing mica pulp comprising the steps of heatingmica to approximately 800 C. for sufficient time partially to open upthe leaves thereof, introducing a substantially cooler gas into theinterstices between said leaves formed by said heating step, confiningsaid gas between said leaves by immersing the same in a liquid, andthereafter further exfoliating said mica in said liquid by agitating thesame and by expanding said confined gas by lowering the pressure on thesurface of said liquid to a value substantially lower than the pressureof said gas between said leaves after said immersion.

2. A process for producing mica pulp comprising the steps of heatingmica to approximately 800 C. for sufficient time partially to open upthe leaves thereof, introducing air at substantially room temperatureinto the interstices between said leaves formed by said heating step,confining said gas between said leaves by immersing the same in aliquid, and thereafter further exfoliating said mica in said liquid byagitating the same and by expanding said confined air by lowering thepressure on the surface of said liquid to a value substantially lowerthan the pressure of said air between said leaves after said immersion.

3. A process for producing mica pulp comprising the steps of heatingmica to approximately 800 C. for sufficient time partially to dehydratethe same and to open up the leaves thereof, introducing air atsubstantially room temperature into the interstices between said leavesformed by said heating step, confining said gas between said leaves byimmersing the same in a liquid, and thereafter further exfoliating siadmica in said liquid by agitating the same and by expanding said confinedair by lowering the pressure on the surface of said liquid to a valuesubstantially lower than the pressure of said air between said leavesafter said immersion.

4. In a process for producing mica pulp, the combination of stepscomprising, opening up and partially dehydrating mica by heating it toapproximately 800 C., thereafter introducing gas between the leaves ofsaid mica at a temperature and pressure suitable for subsequentexpansion of said gas while said mica is immersed in a liquid, and thenfurther exfoliating said mica by simultaneously agitating the same in aliquid and expanding 7 said gas.

Heard et a1. Nov. 19, 1901 Bardet Apr. 24-, 1951

1. A PROCESS FOR PRODUCING MICA PULP COMPRISING THE STEPS OF HEATINGMICA TO APPROXIMATELY 800*C. FOR SUFFICIENT TIME PARTIALLY TO OPEN UP TOLEAVES THREOF, INTRODUCING A SUBSTANTIALLY COOLER GAS INTO THEINTERSTICES BETWEEN SAID LEAVES FORMED BY SAID HEATING STEP, CONFININGSAID GAS BETWEEN SAID LEAVES BY IMMERSING THE SAME IN A LIQUID, ANDTHEREAFTER FURTHER EXFOLIATING SAID MICA IN SAID LIQUID BY AGITATING THESAME AND BY EXPANDING SAID CONFINED GAS BY LOWERING THE PRESSURE ON THESURFACE OF SAID LIQUID TO A VALVE SUBSTANTIALLY LOWER THAN THE PRESSUREOF SAID GAS BETWEEN SAID LEAVES AFTER SAID IMMERSION.